Radioactive decontamination

ABSTRACT

A method for the removal of embedded contamination from a metallic surface in which a laser beam is directed on to the contaminated surface. The laser beam has sufficient power density to cause direct ejection of laser-generated melt pool liquid from the metallic surface thereby removing a metallic surface layer containing the embedded contamination. Means are provided for the collection of laser ejected material in order to prevent recontamination of the metallic surface or contamination of previously uncontaminated surfaces.

This application is a 371 of PCT/G02452 filed Nov. 8, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the removal of radioactive contamination and,more particularly, to the removal of embedded radioactive contaminationfrom metallic surfaces using laser beams.

2. Discussion of Prior Art

During the operation of nuclear processing plants it is inevitable thatsurfaces will become contaminated with radioactive substances.Consequently, during the decommissioning of these plants it is necessaryto decontaminate the contaminated surfaces in a safe manner. Often thecontaminated surfaces comprise stainless steels or mild steels andtypical contaminants include UO₂, PuO₂, Co-60, Sr-90, Cs-134 and Cs-137.The contaminants may be in the form of fine particles or solutions whichcan penetrate into steel substrates for a distance of about 4 mm. Insuch situations well known decontamination techniques such as chemicalwashing, fluid shear blowing or paste/stripping are not effective forthe removal of embedded contamination.

One current approach for the reduction of contamination is to maintain anegative pressure within a nuclear containment such that radioactivecontamination is confined within specific zones. However, such a schemehas a disadvantage in that running costs are high.

EP0091646 describes a technique for laser (ns pulse)ablation/vaporisation of thin (less than 40 microns) metal oxide filmsfrom metal surfaces. The ablation technique is achieved by applying ahigh energy laser pulse (exceeding 1 GW) to directly break molecularbonds without going through thermal stages. The typical depth of theremoved layer is of the order of microns. The laser vaporisation removalis not efficient for metallic surfaces since much heat can be lostthrough conduction. Again the depth of the removed layer is in themicron range.

Another known technique, described in JP 63024139, uses oft axis gasinjection into the laser melt pool for the removal of laser-generatedmolten materials. This technique can achieve the removal of surfacelayers of the order of millimetres. However, the alignment of the gasjet relative to the melt pool is critical and when there are objectstandoff changes the correct alignment is often difficult to achieve.Another disadvantage is that this technique is suited to processing inone direction only.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method for theremoval of embedded contamination from a metallic surface, the methodcomprising directing a laser beam on to the contaminated surface, thelaser beam having sufficient power density to cause direct ejection oflaser-generated melt pool liquid from the metallic surface therebyremoving a metallic surface layer containing the embedded contamination.

Preferably, the power density is greater than 6 MW/cm².

Preferably, the laser beam comprises pulsed energy, eg having a pulselength of at least 1 ms and a pulse energy of 5 J.

The method makes use of laser-generated vapour pressure and opticalpressure to achieve the direct ejection of laser molten liquid, and thelaser generated vapour recoil pressure is typically between 5 to 100bar. The molten liquid can be ejected at least 0.1 metre and as far as2.5 metres from the melt pool.

Conveniently, the metallic surface may comprise stainless steel or mildsteel.

Advantageously, the ejection of the laser-generated melt pool liquid isachieved without the use of an additional gas jet blown into the meltpool.

The method can remove a contaminated surface layer to a depth of up to 5mm.

Desirably, means may be provided for the collection of laser ejectedmaterial in order to prevent recontamination of the metallic surface orcontamination of previously uncontaminated surfaces and the collectionmeans may comprise an air/water spray and an extraction system.

The laser producing the laser beam may be a gas or a solid state typelaser.

The inventors have recognised that since the majority (more than 90%) ofembedded contamination is within 1 mm of the surface of contaminatedsteel, the removal of this surface layer allows the level ofcontamination to be greatly reduced. The present invention is, thereforeparticularly advantageous in the safe removal and collection of suchembedded contamination.

The present invention is particularly suited to the removal ofcontamination along a linear path such as that defined by joints,cracks, edges, corners, gaps or the like from which the contaminationcannot be washed out or removed by conventional means during thedecontamination of metallic nuclear installations.

The present invention may also be used for the removal of contaminationfrom the interior surfaces of metallic pipes or tubes.

The meltpool as produced by the method according to the presentinvention is strongly radiation-emitting and we have found that theradiation emitted can be detected, digitised and analysed in the methoddescribed in a copending International Patent Application of even dateby the present applicants claiming priority from GB 9323054.8 thecontents of which are incorporated herein by reference. The imageproduced thereby gives information about the surface orientation, localgeometry and standoff distance relative to the heat or laser sourceproducing the meltpool.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a side elevation showing laser-generated liquid ejectionof-materials from a stationary workpiece;

FIG. 2 is a side elevation showing laser-generated liquid ejection ofmaterials from a moving workpiece;

FIG. 3 is a side elevation showing laser-generated liquid ejection ofmaterials and a collection means;

FIG. 4 is a side elevation showing laser-generated liquid ejection ofmaterials and an alternative collection means;

FIG. 5 is a graph of metal removal depth versus laser pulse length;

FIG. 6 is a graph of metal removal depth versus laser pulse energy, and

FIG. 7 is a graph of melt depth versus laser traversing speed.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, a laser beam 2 is shown impinging upon asurface 4 of a stationary metallic workpiece 6, the surface 4 having alayer of embedded radioactive contamination 8. The laser beam 2 has apower density of greater than 6 MW/cm² and is operated at a pulse lengthof several milliseconds. At the point where the laser beam 2 meets thesurface 4 a laser melt pool 12 is formed. Molten material 10, containingthe radioactive contamination 8 is ejected from the melt pool 12 due toa laser-generated vapour recoil pressure of between 5 to 100 bar and toa lesser extent to a laser photo pressure (which is the power densitydivided by the speed of light). The ejected material may be thrown fordistances of up to 2.5 metres from the melt pool 12.

In FIG. 2 the laser beam 2 is shown impinging upon the surface 4 of theworkpiece 6 with the workpiece 6 now moving in the direction indicatedby the arrow. As described in relation to FIG. 1, molten material 10containing the radioactive contamination 8 is ejected from the lasermelt pool 12 for distances of up to 2.5 metres. When the workpiece 6 istravelling in the direction indicated by the arrow the molten material10 also tends to be ejected in that direction. In situations where thelaser beam 2 is moving and the workpiece 6 is stationary, the moltenmaterial 10 is ejected in the direction opposite to the direction oftravel of the laser beam 2.

Referring now to FIG. 3, the laser beam 2 is shown impinging upon thesurface 4 of the moving workpiece 6 such that molten material 10 isejected as described in relation to FIG. 2. Prior to impinging upon thesurface 4 the laser beam 2 passes through a collection means 20 locatedin close proximity to the surface 4. The collector 20 comprises ahousing 22 having a laser inlet 24 and a laser outlet 26 aligned suchthat the laser beam 2 passes through the housing 22 in an uninterruptedmanner to impinge upon the surface 4. The housing 22 has two opposedextraction outlets 36, 33 located on an axis of symmetry of the housing22, the axis of symmetry being approximately perpendicular to the laserbeam 2. A nozzle 28 is located in the housing 22 and is positioned so asto point in a direction approximately perpendicular to the laser beam 2.The nozzle 28 is connected via a tube 30 to a compressed air inlet 32and to a water inlet 34. The collector 20 is rotatable by means of amotorised rotational system (not shown).

In operation, the collector 20 moves synchronously with the movement ofthe laser beam 2 and the molten ejected material 10 is sprayed with anair/water mist 40 from the nozzle 28. The molten material 10 is therebycooled to form metallic particles which contain the radioactivecontamination 8. These particles and water are removed from the housing22 via the extraction outlets 36, 38 by suitable extraction means (notshown) acting on the outlets 36, 38. The collector 20 may be rotated bythe motorised rotational system (not shown) so as to allow laserprocessing to occur in all directions.

The use of the water/air mist has been found to be very effective incooling the molten ejected material and thereby facilitates thecollection of the metal particles (which typically may have diameters ofup to 3 millimetres). For stainless steel and mild steel workpieces, thetypical depth from which material is ejected is around 0.5 to 1.5millimetres per pulse (of 1 to 10 milliseconds duration) using a YttriumAluminium Garnet (YAG) laser. The rate of ejection of material from thesurface is between 50 to 100 cm²/kWhr.

In FIG. 4 an alternative collection means 50 is shown comprising ahollow cylindrical housing 52, open at one end and with its axis ofsymmetry perpendicular to the direction of the laser beam 2 and in closeproximity to the surface 4. The housing 52 has a laser inlet 54 and alaser outlet 56 arranged such that the laser beam 2 passes in anuninterrupted manner through the housing 52 to impinge upon the surface4. A nozzle 58 projects into the housing 52 by way of the closed end 60thereof and points along the axis of symmetry of the housing 52 so as todischarge through the laser beam 2. The nozzle 58 is connected via atube to a compressed air inlet 62 and a water inlet 64.

In operation of the collector 50, the molten ejected material 10 issprayed with an air/water mist 66 from the nozzle 58. The moltenmaterial is thereby cooled to form metallic particles which contain theradioactive contamination 8. The particles and water are removed fromthe housing 52 by suitable extraction means (not shown) acting on theopen end of the housing 52.

The use of the collectors described above allows contaminated material,removed by direct ejection of laser molten material from the surface, tobe collected and removed so that the decontaminated surface is notrecontaminated by molten contaminated material depositing on thedecontaminated surface.

FIGS. 5 to 7 show the relationships between a number of operatingparameters and material removal depth for the method described above,when using a YAG laser operating at between 10 to 55 Joules, with a 1 to8 millisecond pulse time, having a repetition rate of between 3 to 30Hertz and a laser spot size of about 1 millimetre diameter.

FIG. 5 is a graph of depth of removed material versus length of laserpulse, FIG. 6 is a graph of depth of material removed versus energy ofthe laser pulse and FIG. 7 is a graph showing the depth of moltenmaterial versus the traversing speed of the laser beam. From theserelationships it can be seen that a minimum power energy and interactiontime are required to initiate the molten liquid ejection. Too high aninteraction time would be less efficient since some energy would be lostby conduction and heating of vaporised material. There is an optimumenergy and interaction time which have quadrant relationships with theremoval depth, the removal depth being largely controlled by pulse widthand energy density. Traversing speed of the laser beam has very littleeffect on the depth of material removal. However, when the traversingspeed is too low, low height sputtering takes place due to repeatedheating of the same spot through reduced laser power density (beamdefocus) at a certain depth which can generate volcano-like craters anddebris. Too high a traversing speed tends to produce discontinuousremoval of material. An optimum traversing speed has been found to beapproximately equal to the laser spot size multiplied by the laser pulsefrequency. Therefore, a high laser beam repetition rate would enable ahigh processing speed.

Compared to other laser decontamination methods, laser generated liquidejection is more economic in terms of gas saving. The use of acompressed air/water mist (at an air flow rate of less than 500 litresper minute and a water flow rate of 0.2 litres per minute) enables thecooling and collection of the ejected material to be achieved in asingle process.

What is claimed is:
 1. A method for the removal of embeddedcontamination from a metallic surface, the method comprising directing alaser beam on to the surface, the laser beam having sufficient powerdensity to melt at least a portion of said surface and to cause directejection of laser-generated melt pool liquid from the metallic surfaceby laser-generated vapor pressure in the melt pool liquid, therebyremoving a portion of said metallic surface layer containing theembedded contamination, said laser beam having a pulse duration of atleast 1 millisecond.
 2. A method as in claim 1 and wherein the directejection of the laser-generated melt pool liquid is achieved without theuse of an additional gas jet blown into the melt pool.
 3. A method as inclaim 1 and wherein a laser-generated recoil pressure is between 5 to100 bar.
 4. A method as in claim 1 and wherein the laser beam powerdensity is greater than 6 MW/cm².
 5. A method as in claim 1 and whereinthe laser producing the laser beam is a gas or a solid state laser.
 6. Amethod as in claim 1 and wherein the metallic surface comprisesstainless steel or mild steel.
 7. A method as in claim 1 and whereinmeans are provided for the collection of laser ejected material in orderto prevent recontamination of the metallic surface or contamination ofpreviously uncontaminated surfaces.
 8. A method as in claim 7 andwherein the means provided for the collection of laser ejected materialcomprise an air/water spray and an extraction system.
 9. A method forthe removal of embedded contamination from a metallic surface, themethod comprising directing a laser beam on to the surface, the laserbeam having sufficient power density to cause direct ejection oflaser-generated melt pool liquid from the metallic surface therebyremoving a portion of said metallic surface layer containing theembedded contamination, wherein the direct ejection of thelaser-generated melt pool liquid is achieved without the use of anadditional gas jet blown into the melt pool.
 10. A method for theremoval of embedded contamination from a metallic surface, the methodcomprising directing a laser beam on to the surface, the laser beamhaving sufficient power density to melt at least a portion of saidsurface and to cause direct ejection of laser-generated melt pool liquidfrom the metallic surface by laser-generated vapor pressure in the meltpool liquid, thereby removing a portion of said metallic surface layercontaining the embedded contamination.